Recently, wireless LAN systems, in which the IEEE (Institute of Electrical and Electronics Engineers) 802.11b and IEEE802.11a are typical systems, are widely used. In the wireless LAN systems of the IEEE802.11 type, an autonomous decentralized system using a CSMS/CA (Carrier Sense Multiple Access/Collision Aviodance) scheme as an access scheme is used (for example, refer to a patent reference 1).
That is, FIG. 9 shows a protocol stack of the lower two layers in an OSI (Open System Interconnection) model of an IEEE802.11-type wireless LAN system, which is composed of a data link layer 901 and a physical layer 902.
The data link layer 901 is divided into a LLC (Logical Link Control) sublayer 903 and an MAC (Media Access Control) sublayer 904. In the IEEE wireless LAN system, a logical link control 905 under the IEEE802.2 is defined as the LLC sublayer 903, and the CSMA/CA scheme under the IEEE802.11 is defined as the MAC sublayer 904.
In addition, the physical layer 902 has two definitions, an IEEE802.11b (907) of a direct sequence type using a coding technique called a CCK (Complimentary Code Keying) in 2.4 GHgz band, and an IEEE802.11a (908) using an OFDM (Orthogonal Frequency Division Multiplex) technique in 5 GHz band.
FIG. 10 shows transmission/reception timings in a wireless LAN system using the CSMA/CA scheme of the IEEE802.11. This wireless LAN system is composed of a base station and first to fourth terminal stations, and the first, second and third terminal stations operate in a normal transmission/reception mode and the fourth terminal station operates in an intermittent reception mode.
The base station basically transmits beacons (BC1, BC2, BC3, . . . ) which are used for establishment of synchronization between the terminal stations and for notification of various control information at an interval of a beacon time period Ti. This beacon time period is divided into a period for non-conflict due to polling from the base station and a period for conflict due to conflict (conflict) between the terminal stations. The non-conflict period is notified by a beacon.
During the non-conflict period, only a terminal station specified by polling transmitted from the base station is allowed to perform transmission. For example, polling PL1 is polling for the first terminal station, and the first terminal station transmits a data packet DP1 in response to the reception of the polling PL1. The base station returns an ACK (acknowledgement) signal AC1 when receiving the data packet DP1 correctly. Similarly, polling PL2 is polling for the second terminal station, and the second terminal station transmits a data packet DP2 in response to the reception of the polling PL2. The base station returns an ACK signal AC2 when receiving the data packet DP2 correctly. As to data such as an AV stream which requires real-time property, it is periodically transmitted and received under the control of the base station during the non-conflict period, thus keeping QoS (Quality of Service) of transmission.
During the conflict period, on the contrary, each terminal station receives an ACK signal, and performs carrier sense after waiting for a waiting time based on a corresponding random number (hereinafter, referred to as random backoff time), and performs transmission while the other terminal stations do not perform transmission.
That is, when the first and second terminal stations having data which should be transmitted receives the ACK signal AC2, they carries out the carrier sense after waiting for the random backoff times t1 and t2, respectively. Since the random backoff time t1 of the first terminal station is shorter than the random backoff time t2 of the second terminal station, the first terminal station succeeds the carrier sense and transmits a data packet DP3. The base station returns an ACK signal AC3 for the success of the reception of the data packet DP3. The second terminal station which did not succeed the carrier sense performs the carrier sense again after the reception of the ACK signal AC3, and transmits a data packet DP4.
On the other hand, the fourth terminal operating in the intermittence reception mode receives each beacon transmitted from the base station by performing the intermittence reception at an interval of a beacon time period Ti by synchronizing with the base station. In a case where a beacon includes a call for the own station, the fourth terminal station stops the intermittence reception mode and goes back to the normal transmission/reception mode.
Patent reference 1 Japanese Patent Laid Open 11-74886 (the second to third pages, FIG. 11 and FIG. 12)
Here, due to transmission of a data packet from a terminal station in aforementioned CSMS/CA scheme of the IEEE802.11, transmission of a beacon from the base station may be delayed.
That is, referring to FIG. 10, the third terminal station transmits a data packet DP5 in the last of the conflict period of the beacon time period Ti starting with a beacon BC2. Since this data packet DP5 is transmitted over the beacon time period Ti, the base station has to receive the data packet DP5 to the last, and as a result, it can not transmit a beacon BC3 which was to be transmitted after the beacon time period Ti of the beacon BC2.
As a result, the base station transmits a beacon BC4 at a timing delayed from the beacon time period Ti by a delay time Δt, and therefore a beacon interval lengthens by the delay time Δt. This means that the start timing of a next non-conflict period is delayed, and thus the QoS during the non-conflict period can not be secured, which is a problem.
In addition, the fourth terminal station operating in the intermittence reception mode performs the intermittence reception of beacons at an interval of the beacon time period Ti by synchronizing with the base station. Therefore, if the beacon interval lengthens, a timing of the intermittent reception becomes different from a timing of transmission of a beacon and thus the fourth terminal station operating in the intermittence reception mode can not receive the beacon, which is a problem.